In case they already have their set of depleted uranium dice...
Mammoth Ivory D20s, made from the tusk of real-life extinct pachyderms.
Apparently the first run of 24 is sold out, but more ivory is said to be procured for another batch.
You, too, can roll criticals with the teeth of a beast that failed its save vs. extinction.
(From Artisan Dice - makers of insane art-dice)